In fluidized bed boilers, various ways of supplying air are used, the aim being that the fuel would burn as efficiently as possible and yet the combustion process could be controlled in a desired manner in both the horizontal and the vertical directions of the boiler. Typically, air is supplied from one or more levels in the vertical direction of the fluidized bed boiler so as to cause sub-stoichiometric combustion in the gas flow direction as far as possible, i.e. in the vertical direction of the fluidized bed boiler. The final air causing stoichiometric combustion is not fed until the final, typically tertiary step. To fluidize the material in the fluidized bed, primary air is supplied from below the fluidized bed through a grate so as to achieve the desired fluidizing level and the desired circulation of the fluidized bed material in the fluidized bed.
To make the combustion efficient, the fuel and the combustion air must be made to mix as well as possible. It has-been noted, however, that the fluidizing air coming from below the grate allows the fine fuel particles to move with the gas flow to the upper parts of the furnace, which defers the combustion step so much that the combustion is no longer efficient and the emissions are not reduced efficiently. The most advantageous solution with regard to the emissions would be if the combustion were sub-stoichiometric as far as possible, so that essentially no No.sub.x compounds would be formed. The fact that the fuel particles move up with the gas flow and burn there may make the temperature close to the superheaters rise too high, which speeds the corrosion of the superheaters and thereby shortens their effective life. Problems are posed by the channelling of the flows in the upper part of the furnace and by different vertical backflows, whereby the volume of the furnace is not actually used efficiently with respect to the reactions, and so the walls. cannot be used efficiently for heat transfer.